Plasma pinch X-ray method

ABSTRACT

A method is provided for producing plasma pinch X-rays usable in X-ray lithography. Ionized heated plasma is repeatably generated in a first area directly from solid material without exploding the latter. X-rays are generated in a second area by passing high current through the plasma causing radial inward magnetic field pinching. Accurate control and improved intensity performance, and greater flexibility in selection of X-ray emitting materials, are provided by the separation of the plasma generating and the X-ray pinch generating functions.

This is a division of application Ser. No. 420,558, filed Sept. 20,1982, now U.S. Pat. No. 4,536,884.

TECHNICAL FIELD

The invention relates to systems for generating X-rays from plasma, andusable in fine line lithography for semiconductors.

BACKGROUND

The invention arose from efforts to develop an X-ray system for use inmanufacturing microelectronic circuits. With the ever increasingminiaturization of semiconductor integrated circuitry, opticallithography does not afford the necessary resolution due to diffractioneffects along mask lines. X-ray lithography provides greater resolutiondue to the shorter wave length of X-rays.

Various types of X-ray sources are known for use in X-ray lithography.These systems are costly, and have not yet achieved a consistently highlevel of performance and intensity necessary for high production ratelithography. Various of these systems are cumbersome, and are notamenable to repetitive manufacturing sequences.

In the conventional type of X-ray system, a metal target is continuouslybombarded by a stream of high energy electrons. Most of the energy isdissipated in the target in the form of heat, while a very smallfraction is emitted in the form of relatively high energy X-rays. Thistype of source system has low intensity and low production rates. Thehigh heat generation requires complicated mechanical designs todissipate the heat, such as rotating anodes or high velocity watercooling.

In another type of X-ray system, commonly called the gas puff type, aneutral non-ionized gas is pumped in cylindrical form between a pair ofelectrodes. High current is then passed between the electrodes, whichheats and ionizes the gas, thus creating plasma. The high current alsocauses magnetic field pinching of the plasma to a smaller constrictedarea, i.e. parallel lines of current create magnetic fields which causeattraction of the current lines towards each other. The magnetic fieldpinching and compression of the plasma further heats the plasma andcauses X-ray emission.

In an alternate gas puff type X-ray system, the cold, neutral gas ispre-ionized, for example electrically, or by radio frequency radiationsetting up a standing wave which ionizes the gas. This alternate gaspuff system affords better performance, but is extremely costly.Mechanical valving or the like is eeded for introduction of the gas, asin the original gas puff system, and there is required the additionalequipment for the intermediate pre-ionization stage. Also as in theoriginal gas puff system, the X-ray generating material selection islimited by the requirement that the material be a gas.

In another known X-ray system, called the exploding wire type, highcurrent explodes and vaporizes a circumferential array of wires to avapor plasma. The high current also causes magnetic field pinching ofthe vapor plasma, generating X-rays. The same current which generatesthe plasma also generates the X-rays. The plasma and the X-rays aregenerated in the same area and at the same time. A drawback of theexploding wire type X-ray system is its one shot nature. The wires mustbe replaced after each firing, and the system is thus not amenable tocost effective use in manufacturing sequences.

In another type of X-ray system, as shown in McCorkle U.S. Pat. No.4,201,921, plasma is generated by passing a high current along the innercapillary wall of a hollow tubular insulator. X-rays are generated bydirecting an electron beam on the plasma.

SUMMARY

The present invention provides a simple, low cost X-ray system. Thoughnot limited thereto, the invention is particularly suitable for use inX-ray lithography, including cost effective repetitive manufacturinguse.

The invention has been found to afford remarkably superior performanceand X-ray intensity. The invention affords plasma pinch X-rays in aparticularly simple and effective system. In preferred form, X-raygeneration and plasma generation are separated in time and space, andhave separate initiating means. Ionized heated plasma is repeatablygenerated in a first area directly from solid material, as by highcurrent spark discharge from a conductor, or by high current strippingalong an insulator. X-rays are generated in a second different area at asecond later time and by a different high current causing magnetic fieldpinching of the plasma.

The system enables accurate control of both plasma generation and X-raygeneration. Both are controlled electrically, without mechanical valvingand the like as in gas puff type systems. The use of solid material forX-ray generation enables a wide selection of X-ray source material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic exploded pictorial view of the X-ray system inaccordance with the invention.

FIG. 2 is a schematic sectional view of the X-ray system of FIG. 1.

DETAILED DESCRIPTION

X-ray system 2 in FIG. 1 includes plasma vapor generating means 4 andX-ray generating means 6. The plasma is generated in a first area 8 bypassing high current along solid material means 10. In preferred form, aplurality of circumferentially spaced solid material means 10 afford acylinder of plasma at an annulus area 8.

In one form, each of the solid material means 10 comprises an electrode12 connected through a ballast resistor 14 to a source of electricalenergy, such as a large storage capacitor 16 charged from voltage source17. Upon closure of switch 18, capacitor 16 will discharge fromelectrode 12 across gap 8 to electrode 20. This spark discharge fromelectrical conductor 12 boils material off the latter to generate plasmain the arc gap 8. Generation of plasma by spark discharge is known. Inanother form, each of the solid material means 10 further includeselectrical insulator means 22 having a central axial passage 24 in theform of a hollow tubular capillary. High current flows from electrode 12along the inner surface wall of capillary 24, FIG. 2, and strips orboils material off the latter, generating plasma. Plasma generation byhigh current stripping along an insulator is known. Alternatively,material 22 may be a conductor, or a semiconductor, e.g. silicon.

The ionized heated plasma in first area 8 is cylindrically communicatedthrough passage 26 to the second area 28. Cylindrical passage 26 throughelectrode 20 is coaxially aligned with the plurality ofcircumferentially spaced solid material means 10, such that the plasmais cylindrically generated and communicated. X-ray generating means 6includes an electrode 30 axially aligned with cylindrical passage 26 andaxially spaced from electrode 20 on the opposite side thereof fromplasma generating means 4. Electrode 30 may take a variety of forms,including a solid electrode, or a wire mesh screen. Upon closure ofswitch 32, a large storage capacitor 34 discharges electrode 30 acrossgap 28 to electrode 20. Capacitor 34 is charged by voltage source 35.This passes high current through the plasma in area 28 causing magneticfield pinching of the plasma radially inwardly. Electrode 20 thusprovides a common electrode for plasma generating means 4 and X-raygenerating means 6.

Common electrode 20 includes a second passage 36 extending therethroughalong the central axis and incentrically to cylindrical passage 26.Passage 36 provides for exit emission of X-rays from second area 28 backthrough common electrode 20 in a direction opposite to plasma flow.X-rays are emitted as shown at arrow 38.

The plasma may be provided to second area 28 in the form of a solidcylinder, but a hollow cylinder is preferred in order to facilitateeasier pinching. In the preferred structure, this hollow cylinder ofplasma is provided through cylindrical passage 26. The central electrodeportion 40 is supported within passage 26 by radial spokes such as 42,FIG. 2. These spokes also provide the ground return for central portion40.

In the preferred embodiment, the plasma generating means 4 includes sixelectrodes 12, circumferentially arranged. Insulating material 22 may beteflon having a length of 0.5 cm and an inner diameter of 1 mm. Hollowcylindrical passage 26 in common electrode 20 has a length of 1 cm, aninner diameter of 4 cm and an outer diameter of 4.6 cm, thus havingradial thickness of 3 mm. The central X-ray emission passage 36 has adiameter of 2 mm. The axial gap across area 28 between electrode 20 andelectrode 30 is 1 cm. The plasma is pinched radially inwardly to adiameter of about 1 mm in area 28 coaxially aligned with passage 36. Themagnetic field pinch collapse occurs within about 50 nanoseconds. Thesize of capacitors 16 and 34 is 0.2 uf and 0.7 uf respectively. X-raygenerating electrode 30 is fired 2 microseconds after plasma generatingelectrodes 12 are fired.

It is thus seen that in the preferred embodiment, the plasma and theX-rays are generated in different areas and at a different time and withseparate currents. The plasma is generated in first area 8, and X-raysare generated in a second area 28. The X-rays are generated sequentiallysubsequent to generation of the plasma. A first current from electrodes12 generates the plasma, and a second separate current from electrode 30generates the X-rays. Ionized heated plasma is repeatably generated inarea 8 directly from solid material without exploding the latter.

X-rays are radiated in all directions from pinch area 28. The X-rays arepreferably communicated along axis 38 through exit passage 36. Anotherpreferred path for X-ray emission is in the opposite direction alongaxis 38 through an aperture in electrode 30. The axial emissionorientation of the X-rays enables placement of a masked semiconductorsubstrate 50 normal to axis 38 for impingement by the X-rays.

It is preferred that plasma vapor be generated in area 8, thoughalternatively other lower energy level vapors may of course be generateddirectly from the solid material. In a further alternative, when plasmavapor is generated in area 8, the delay or length of travel to area 28may be long enough that the plasma cools slightly but is still a vapor.In both alternatives, the high current through area 28 heats the vaporup to plasma vapor and causes magnetic field plasma pinching to generatethe X-rays.

It is recognized that various modifications are possible within thescope of the appended claims.

We claim:
 1. A method of producing X-rays comprising:generating vapordirectly from solid material in a first region; communicating said vaporto a second region in spaced axial paths; and generating X-rays in saidsecond region by passing a high current axially through said vaporcausing magnetic field plasma pinching radially inwardly; repeatablygenerating plasma vapor directly from said solid material by passinghigh current along said solid material, said plasma generating currentbeing separate from said X-ray generating current.
 2. A method ofproducing X-rays comprising:generating vapor directly from solidmaterial in a first region; communicating said vapor to a second regionin spaced axial paths; and generating X-rays in said second region bypassing a high current axially through said vapor causing magnetic fieldplasma pinching radially inwardly; generating said vapor by passing ahigh current along said material, said last mentioned high current beingseparate from said first mentioned high current.
 3. The inventionaccording to claim 2 wherein said current generating said vapor is priorin time to said current generating said X-rays.
 4. A method forproducing X-rays, comprising:repeatably generating vapor directly fromsolid material in a first region by passing current along solid materialmeans; communicating said vapor along spaced axial paths to a secondregion; generating X-rays in said second region by passing a highcurrent axially through said vapor causing magnetic field plasmapinching radially inwardly; communicating said vapor in the form of ahollow axially extending cylinder comprised of a plurality of axialpaths spaced along a circumference, and passing said high currentaxially through said vapor in said second region along said cylinder toafford said radial inward pinching.
 5. The invention according to claim4 comprising cylindrically generating said vapor coaxially to saidcylinder.
 6. The invention according to claim 5 comprising communicatingsaid X-rays along an axial path spaced radially inwardly of said hollowvapor cylinder and in the same direction as vapor flow.
 7. The inventionaccording to claim 5 comprising communicating said X-rays along an axialpath spaced radially inwardly of said hollow vapor cylinder and in theopposite direction to vapor flow.
 8. A method for producing x-rayscomprising in combination:repeatably generating ionized heated plasmadirectly from solid material; generating X-rays by passing high currentthrough said plasma causing magnetic field pinching; generating saidplasma and said X-rays separately in time, and generating said plasmaand said X-rays separately in space by generating said plasma in a firstregion and generating said X-rays in a second region by passing highcurrent axially through said plasma causing radial inward magnetic fieldplasma pinching perpendicularly to the axial current flow, andgenerating said plasma with a first current source and generating saidX-rays with a second current source separate and distinct from saidfirst current source, affording accurate electric control of both plasmageneration and X-ray generation.